Community Antenna Television ("CATV") systems are used in a widespread manner for the transmission and distribution of television signals to end users, or subscribers. In general, CATV systems comprise a transmission subsystem and a distribution subsystem. The transmission subsystem obtains television signals associated with a plurality of CATV channels and generates a broadband CATV signal therefrom. The distribution subsystem then delivers the CATV broadband signal to television receivers located within the residences and business establishments of subscribers. The complexity and size of the distribution subsystem requires that operation and performance be periodically tested and/or monitored.
One test often performed by CATV service providers in order to pinpoint problems in the distribution subsystem is fault detection. Fault detection refers to the process of locating faults within the distribution subsystem such as breaks, shorts, discontinuities, degraded components, and improperly terminated transmission lines. Faults within the distribution subsystem are typically characterized by an impedance mismatch. In other words, the impedance of the fault is typically different than the characteristic impedance of the transmission lines of the distribution subsystem. For example, transmission lines in a CATV distribution subsystem typically have an impedance of approximately 75 ohms; however, a short on the transmission line would have an approximately zero impedance and a break would have an approximately infinite impedance.
One problem with faults in the distribution subsystem is that faults, due to their impedance mismatch characteristics, reflect signals transmitted through the distribution subsystem. As a result, beyond cutting off portions of the distribution subsystem in the case of a short or a break, faults in the distribution subsystem may also cause problems throughout the distribution subsystem due to interference from reflected signals. Therefore, it is important for CATV service providers to be able to locate faults within the network in order to repair the fault to not only cure reception problems of a single subscriber but to remove fault generated interference from the distribution subsystem as a whole. While impedance mismatches within the distribution subsystem may interfere with CATV signals, some impedance mismatches due to their severity may not generate enough interference to justify the cost of repair. As a result, CATV service provides also need information concerning the severity of the impedance mismatch.
One way of determining location and severity of faults within the distribution subsystem is to perform frequency domain reflectometry upon the distribution subsystem. Frequency domain reflectometry utilizes a reflectometer that applies a sweep signal to the distribution subsystem. The sweep signal is an RF signal that is swept from a start frequency to a stop frequency. If an impedance mismatch exists within the distribution subsystem, the impedance mismatch will reflect each transmitted signal back to the reflectometer at the same frequency as the transmitted signal but retarded in phase. As a result of this reflection, a standing wave is generated. The reflectometer measures the level of the standing wave at each swept frequency in order to obtain a reflected sweep response signal. The retardation of the reflected sweep response signal is such that the minimums of the reflected wave will align to 1/2 the wavelength of the impedance mismatch from the reflectometer. Due to this known relationship, the reflectometer may determine the distance from the reflectometer of the impedance mismatch.
To this end, the reflectometer may perform spectral analysis upon the reflected sweep response signal in order to obtain a reflected sweep response spectrum having a plurality of spectral peaks. Each spectral peak includes a frequency and a magnitude that together represent a sinusoidal component of the reflected sweep response spectrum. The reflectometer may apply the above wavelength relationship to the frequency of a spectral peak in order to determine distance from the reflectometer of an impedance mismatch within the transmission line. Furthermore, the reflectometer may determine severity of the impedance mismatch from the magnitude of the spectral peak.
One problem associated with the above reflectometry system is the failure to account for loss due to attenuation within the transmission line. Transmission lines generally dampen the magnitude of sinusoidal signals that are traveling through the transmission line as a function of distance and as a function of the square root of the frequency. As a result of this dampening, the magnitude of the spectral peak should actually be greater in magnitude in order to accurately represent the severity of the impedance mismatch. Therefore, if the reflectometer fails to account for this loss due to attenuation, the reflectometer may indicate that an impedance mismatch is not severe enough to require repair even though the impedance mismatch actually is severe enough.
Accordingly, there is a need for a reflectometer that accounts for attenuation effects when determining severity of impedance mismatches within a transmission line. However, the effect of attenuation on the magnitude of a spectral peak has no closed form. As a result, determination of an appropriate attenuation compensation factor for a spectral peak may be computationally intensive. In order to support a computationally intensive function, a reflectometer may need advanced processing circuitry thus adding to the cost of the reflectometer. Accordingly, there is also a need for a reflectometer that determines an appropriate attenuation compensation factor in a non-computationally intensive manner.